Experiments are proposed to develop a molecular genetic system in Drosophila for regulating the activity of transgenes in both time and space. This technology is particularly needed for use in conjunction with forward genetics to determine the developmental time and the tissue-specific expression pattern required for any given gene. It is of particular interest for the molecular genetic analysis of behavior. An innocuous inducing reagent will be used to turn on the activity of a specific transcription factor expressed transgenically behind cell-type specific promoters. Other transgenes made to be responsive to the transcription factor will therefore be turned on by the inducer in only those cells in which the transcription factor is expressed. Experiments are proposed to test the system by rescuing a Drosophila memory mutant by expressing the normal gene carried on a transgene in both time and in space. Furthermore, lethal mutants in genes potentially important for learning will be rescued by providing expression of the normal gene only during development. The techniques proposed here offer a general method for turning on gene activity both spatially and temporally in Drosophila.